In certain types of equipment it is highly desirable to provide isolation from vibrations produced through operation of the equipment. The helicopter is an example of such a system. Predominant vibratory forces set up by the rotor and transmission assemblies are produced at frequencies which are proportional to the number of rotor blades and to twice the number of rotor blades. Thus, in a helicopter having a four blade rotor, most pronounced vibrations occur at the so-called four per rev and eight per rev levels. Since rotor velocity remains substantially constant for all modes of operation, these vibrations are of constant frequency and in a typical system occur, for example, at frequencies on the order of 20 Hz and 40 Hz, respectively.
It has heretofore been known that an effective way of isolating a body from a mass vibrating a substantially constant frequency is to spring couple the vibrating mass to the body and to provide an auxiliary mass arranged so that inertial forces generated by vibration-induced displacement of the mass act on the body to substantially cancel the constant frequency vibratory forces transmitted through the coupling spring.
It has, however, been a problem to configure a so-called "nodal" isolator of this type in a form which will be effective in a compact environment, such as a helicopter, where severe restrictions are placed on size, weight and range of available motion. A further difficulty is that this type of isolator has heretofore been effective for only a single frequency of vibration whereby only the basic vibrations, e.g., 4 per rev, have been eliminated.